Programmable phase-change memory elements formed from materials that can be programmed to exhibit at least two detectably distinct electrical resistivities are known in the art. Phase-change materials may be programmed between a first structural phase where the material is generally more amorphous and a second structural phase where the material is generally more crystalline. The term amorphous as used herein, refers to a condition that is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term crystalline, as used herein, refers to a condition that is relatively structurally more ordered than amorphous and has lower electrical resistivity than the same material has in the amorphous phase. Since memory elements made with a phase-change material can be programmed to a high resistance state or a low resistance state by changing the phase of the material, one phase can be used to store a logic 0 data bit, for example, while the other is used to store a logic 1 data bit.
A single pulse of energy referred to as a set pulse can be used to transform a volume of phase-change material from the high resistance, amorphous phase, to the low resistance, crystalline phase. Similarly, a single pulse of energy referred to as a reset pulse can be used to transform the volume of phase-change material from the crystalline phase to the amorphous phase. Each phase is non-volatile, i.e., stable, and has characteristic differences that are measurable, such as the change in resistance previously noted.
Electrical resistivity, however, is only one property that changes with a set or a reset of the phase-change material. For example, optical reflectivity also changes with the phase of the material. These changes result because the amorphous-to-crystalline transition is accompanied by discontinuous changes in the volume, density, thermal expansion co-efficient and other material parameters of the phase-change material. Due to these discontinuous changes in the phase-change material, operating the memory device in a phase-change mode is prone to failures. For example, one potential structural failure resulting from the discontinuous changes of the phase-change material is delamination of the phase-change material from the contacts of a memory device, particularly when operating at high frequencies and with high cycling. These types of problems are typically solved by thermal engineering of the structure of the memory element in an effort to minimize stress during operation. Another design solution is selecting suitable contacts to the phase-change material. Both of these solutions require careful engineering of the boundary conditions and interfaces to be manufactured into the memory element.